The fluctuations of torque, i.e., slot ripple, occur during rotation of a rotor of a motor having a stator with winding slots and a rotor with magnets, and disturb that rotation. This is because the relative position of the rotor and the stator is changed, and thus the magnetic flux density distribution is changed, according to the angular position of the rotor. This causes a minute uneven feed, and therefore, the finished accuracy of a workpiece is reduced when a servo motor having such a structure is used in a drive mechanism for a feed in, for example, a machine tool.